This invention relates generally to printers, plotters, and marking devices, and more particularly, to inkjet printers, plotters and marking devices having multiple printheads for multi-color printing.
Inkjet marking devices typically include one or more inkjet pens mounted on a carriage. Each pen includes a printhead having a plurality of inkjet nozzles. During printing, the carriage moves across a media sheet while the nozzles discharge ink drops. The timing of the ink drop ejection is controlled to precisely place the drops at desired locations.
A typical multi-color inkjet marking device includes two or more inkjet pens with respective printheads. One pen stores black ink, while the others store ink of one or more colors, (e.g., cyan, magenta or yellow). The four inks represent four base colors which are applied to a media sheet to derive any of multiple colors.
The pens typically are mounted in stalls within the carriage. To achieve desired print quality the ink colors need to be precisely placed at desired locations on the media sheet. To do so the pen printheads are to be maintained in precise alignment. The pens typically are loaded and replaced periodically by the end user. As a result, mechanical misalignment is likely to occur. Mechanical misalignment results in offsets of one or more pens' nozzles relative to the other pens' nozzles. This misalignment manifests as a misregistration of the dots forming a print symbol, image, or graphic representation. Other sources of misalignment also may occur due to the speed of motion of the carriage, the curvature of the platen and the spray of the nozzles.
One conventional approach for aligning the pens is to use an optical drop detector. This technique is described in U.S. Pat. No. 4,922,270, issued May 1, 1990 to Cobbs et al., entitled "Inter Pen Offset Determination and Compensation in Multi-Pen Thermal Ink Jet Printing Systems." The optical drop detectors detect the position of each ink drop as it leaves the pen. The system then calculates the point of impact of the drop on the print media. Unfortunately the actual impact point often differs substantially from the calculated impact point due to angularity and mechanical tolerances in the system. Angularity results from the movement of the pen in the scan axis as ink is being ejected. There is a delay between the time that the drop of ink is ejected and the time that the drop impacts the media. This flight time delay causes the drop to traverse an angular path toward the media. Inaccurate correction for this delay distorts the image.
U.S. Pat. No. 5,289,208 issued Feb. 22, 1994 of Robert D. Haselby entitled "Automatic Print Cartridge Alignment Sensor System" discloses a technique in which an optical sensor detects the position of test line segments. Vertical alignment is achieved by printing a plurality of non-overlapping horizontal test line segments. A quad photodiode detector detects the vertical positions of the horizontal test line segments relative to a fixed reference. Horizontal alignment is achieved by printing a plurality of non-overlapping vertical test line segments in a vertical direction. If properly aligned the line segments connect to form a straight line (e.g., for printing 2 line segments the line is 2 line segments long). If misaligned, the line segments form an angled line (e.g., for printing 2 line segments the line is 2 line segments long). A quad photodiode detector detects the horizontal positions of the vertical test line segments to determine if the segments are aligned.
U.S. Pat. No. 5,451,990 discloses a reference pattern for use in performing image registration for multiple inkjet cartridges. The reference pattern includes four test patterns. One test pattern is generated along the scan axis to exercise the pens. It includes an individual section for each color. A second test pattern is used to test for pen offset due to speed and curvature. The second pattern is a bidirectional test pattern in which the cartridge is moved at differing speeds in each direction. A pattern is generated for each color. A third pattern is generated by causing each pen to print a plurality of horizontally spaced vertical bars. The fourth pattern includes five columns of vertically spaced horizontal bars. Each column has three sections. The first section in each column is generated using one color (e.g., cyan). In the second section the same color (cyan) is used in columns one and five. The other colors are used respectively in columns 2-4 (e.g., magenta in column 2; yellow in column 3; black in column 4). In the third section of each column, the first color (cyan) is used. Note that the colors do not overlap in any of the patterns.